Add contributing guide to setup dev environment (#470)

Co-authored-by: Pavel Kulyov <[email protected]>

CONTRIBUTING.md

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+# Setup the environment
+
+1. Clone the repo: `git clone [email protected]:python-lsp/python-lsp-server.git`
+2. Create the virtual environment: `python -m venv .venv`
+3. Activate: `source .venv/bin/activate`
+4. Install an editable installation: `pip install -e .`
+    - This will ensure you'll see your edits immediately without reinstalling the project
+5. Configure your editor to point the pylsp executable to the one in `.venv`
+
+## Trying out if it works
+
+Go to file `pylsp/python_lsp.py`, function `start_io_lang_server`,
+and on the first line of the function, add some logging:
+
+```py
+log.info("It works!")
+```
+
+Save the file, restart the LSP server and you should see the log line:
+
+```
+2023-10-12 16:46:38,320 CEST - INFO - pylsp._utils - It works!
+```
+
+Now the project is setup in a way you can quickly iterate change you want to add.
+
+# Running tests
+
+1. Install runtime dependencies: `pip install .[all]`
+2. Install test dependencies: `pip install .[test]`
+3. Run `pytest`: `pytest -v`
+
+## Useful pytest options
+
+- To run a specific test file, use `pytest test/test_utils.py`
+- To run a specific test function within a test file,
+  use `pytest test/test_utils.py::test_debounce`
+- To run tests matching a certain expression, use `pytest -k format`
+- To increase verbosity of pytest, use `pytest -v` or `pytest -vv`
+- To enter a debugger on failed tests, use `pytest --pdb`